Members of the SLAM family of receptors and their adapters, SAP end EAT-2, play an increasingly important role in adaptive and innate immune responses, particularly because of the finding that mutations in SAP cause XLP-disease. Whereas SLAM (CD150, SLAMF1) was first identified as a self-ligand receptor at the interface between T cells and APCs, there is now ample evidence that SLAM serves several distinct functions, e.g. as the primary receptor for Measles Virus, in early hematopoiesis or in microbicidal functions of macrophages. As our Preliminary Studies show, SLAM-/- macrophages are impaired in their ability to kill the parasite L major and several bacteria due to delayed phagosome maturation and a reduced phagocyte NADPH- oxidase function. Macrophages with a disrupted SlamFS gene, which encodes the receptor BLAME display, by contrast, increased NADPH-oxidase activity and accelerated phagosome maturation. The data therefore strongly suggest that in wt mice BLAME, which does not bind EAT-2, negatively interferes with the same PI3P-dependent pathways, and hence with phagosome maturation and reactive oxygen production in macrophages. Our overall hypothesis is that SLAM by positively regulating production of PI3P in the phagosome membrane controls both phagosome maturation and NADPH-oxidase assembly, while BLAME is a negative regulator of the same processes. Down-regulation of PI3P is a mechanism utilized by M. tuberculosis to halt phagosome maturation in order to survive within the macrophage. We will therefore test the hypothesis that killing of M. tuberculosis is altered in BLAME-, SLAM- and/or EAT-2-deficient macrophages. The experiments proposed in this application are grouped in the following specific aims: SA#1: To test the hypothesis that the cell surface receptor SLAM is a positive regulator regulator of phago-lysosomal fusion and phagosomal NADPH-oxidase function in macrophages. SA#2: To test the hypothesis that that BLAME is a negative regulator of phago-lysosomal fusion and phagosomal NADPH-oxidase function in macrophages. . SA#3: To test the hypothesis that immune responses of SLAM-, EAT-2 and BLAME-deficient macrophages to infection with M. tuberculosis are altered (SA#3). Together the experiments should clarify the regulation of microbicidal mechanisms in macrophages. Their outcomes should suggest therapeutic strategies for specific infectious diseases, e.g. tuberculosis, based upon the mechanisms that are regulated by these receptors in the phagosome.